Electrical discharge lamp

ABSTRACT

An operating circuit for an electrical discharge lamp comprises a ballast inductance, a semiconductor switching device and a capacitance connected in series between a pair of input terminals, a pair of output terminals across which said capacitance is connected, and a control network for triggering the switching device into conduction for a predetermined part of each of at least one set of alternate half cycles of an alternating current supply connected across the input terminals, to provide a voltage across the output terminals for both starting and running a discharge lamp.

United States Patent [1 1 Samuels 11] 3,781,595 Dec. 25, 1973 ELECTRICAL DISCHARGE LAMP [75] Inventor:

Philip Rufus Samuels, London, England [73] Assignee: The General Electric Company [22] Filed:

Limited, London, England May 28, 1971 [2]] Appl. No.: 147,983

[52] US. Cl. 3l5/l0l, 315/247 [51] Int. Cl. 1105b 39/00 [58] Field of Search.. 315/247, 273, 283, 3l5/DlG, 7, 241,243, 209, 240, 244, 362, 255

[56] I References Cited UNITED STATES PATENTS 3,376,472 4/1968 Taylor et a1. 315/362 X 3,383,558 5/1968 Waymouth 315/283 X 3,084,283 4/1963 Grunwaldt 3l5/DlG. 7 2/1966 Wattenbach 315/DIG. 7

3,482,142 12/1969 Cluett et a]. 3 l5/D1G. 7 3,628,086 12/1971 Nuckolls 3,241,039 3/1966 Wilting 315/DlG. 7

Primary Examiner-Nathan Kaufman Attorney-Kirschstein, Kirschstein, Ottinger & Frank 5 7] ABSTRACT An operating circuit for an electrical discharge lamp comprises a ballast inductance, a semiconductor switching device and a capacitance connected in series between a pair of input terminals, a pair of output terminalsacross which said capacitance is connected, and a control network for triggering the switching device into conduction for a predetermined part of each of at least one set of alternate half cycles of an alternating currentsupply connected across the input terminals, to provide a voltage across the output terminals for both starting and running a discharge lamp.

9 Claims, 4 Drawing Figures ELECTRICAL DISCHARGE LAMP This invention relates to operating circuits for electrical discharge lamps.

As is well known, the electrical characteristics of electric discharge lamps are such as to require such lamps to be energised from a power supply via an operating circuit incorporating a ballast, typically an inductance-capacitance arrangement. With conventional discharge lamp operating circuits, the lamp running voltage tends to be considerably lower than the voltage of the power supply. As axresult, many lamps require an operating circuit incorporating bulky, high reactance, step-up transformers, which are expensive, or must be operated from power supplies of higher voltage than the standard domestic mains supply.

It is an object of the present invention to provide an operating circuit for an" electrical discharge lamp wherein this difficulty is alleviated.

According to the invention there is provided an oper ating circuit for an electrical discharge lamp comprising a ballast inductance, a semiconductor switching device and a capacitance connected in series between a pair of input terminals, a pair of output terminals across which said capacitance is connected, and a control network for triggering the switching device into conduction for a predetermined part of each of at least one set oe alternate half cycles of an alternating current supply connected across the input terminals, to provide a voltage across the output terminals for both starting and running a discharge lamp; v

The circuit of the present invention serves as an integrated ballast and starting circuit for connection to individual discharge lamps, more especially such discharge lamps as high and low pressure sodium vapour discharge lamps, mercury vapour discharge lamps and fluorescent discharge tubes, and permits the operation of such discharge lamps directly from standard domestic mains alternating current supplies without necessitating the use of step-upttransformers.

A further advantage of a circuit according to the invention is that it is adapted to produce from the alternating current supply a voltage of approximately square waveform between the output terminals. Since the output voltage of the circuit has a substantially square waveform the circuit according to the invention has an improved power factor compared with conventional discharge lamp operating circuits, since more power is delivered at the instant the discharge lamp strikes.

In one particular arrangement according to the invention the switching'device is connected in series with the ballast inductance between one of the input terminals and one of the output terminals.

In another particular arrangement, the switching device is connected in series with said capacitance across said output terminals.

Preferably the switching device is a bilateral device and is triggered intoconduction by the control network for a predetermined part of each half cycle of the alternating current supply.'

Preferably the semiconductor switching device comprises a bilateral semiconductor controlled rectifier. In a preferred embodiment the switching device comprises a four-layer triode. gate-controlled bilateral switch. I

The controlnetwork for triggering the semiconductor switching device conveniently comprises a resistive voltage divider connected across the input terminals. The triggering voltage of the switching device may be made adjustable by making the position of the tapping point of this voltage divider adjustable.

In a preferred arrangement in accordance with the invention a second inductance of smaller value than the ballast inductance is connected in series with said capacitance across said output terminals, thereby to limit the peak value of the current supplied to the lamp in operation. To provide a higher starting voltage with this arrangement, a third inductance inductively coupled with said second inductance may be connected between an intermediate point of said ballast inductance/- switching device/capacitance series combination, and one of the input terminals. The third inductance is suitably connected in series with a second capacitance to form a resonant network having a resonant frequency higher than the alternating current supply.

The third inductance conveniently comprises -a few turns of wire wound around a winding constituting the second inductance.

The invention will be more clearly understood from the following description, given by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 is a prior art circuit diagram of a discharge lamp operating circuit;

FIG. 2 is a circuit diagram of a first preferred embodiment of a discharge lamp operating circuit according to the invention;

FIG. 3 is a circuit diagram illustrating a modification of the circuit of FIG. 2; and

FIG. 4 is a circuit diagram of a second preferred embodiment of a discharge lamp operating circuit according to the invention.

FIG. 1 shows a prior art basic discharge lamp operating circuit including a semiconductor switch TR, arranged in series with a ballast choke L, between an input terminal I, and an output terminal 0, of respective pairs of input and output terminals 1,, I and 0,, O Acapacitor C,'is connected across the input terminals 1,, I to act mainly as a power factor correction capacitor. A capacitor C is connected across the output terminals 0,, 0 forming a resonant network with the ballast choke L,. The output terminal 0 is connected to the input terminal 1 so that the capacitor is connected in series with the choke L, and the switching device TR,, between the input terminals 1,, 1,.

The semiconductor switching device TR, comprises a bilateral semiconductor controlled four layer triode, or Triac, thus having some avalanche of zener properties. The gate electrode of the switching device TR, is connected to an intermediate point of a resistor chain R,, R R, which constitutes a simple voltage divider control network for the switching device TR the resistor chain R, R being connected across the input terminals I,, 1

In operation of the circuit, the input terminals I,, I are connected. across an alternating current single phase mains supply (e.g. 240 volts at 50 Hz) and a discharge lamp D to be operated, for example a high pressure sodium vapour discharge lamp, is connected across the output terminals 0,, O

The semiconductor switching device TR, is arranged, by prior adjustment or selection of the components of the resistor chain R,, R R to be triggered into conduction when the voltage thereacross in either direction reaches a predetermined part of each half cycle of the alternating current supply.

Considering first the operation of the circuit with the lamp D disconnected from the output terminals 0,, 0,, during each half cycle, at the instant of triggering the switching device TR, into conduction a resonant charging voltage of a higher frequency and amplitude than the alternating current supply is produced across the capacitor C by resonance in the network L,, C Thus the voltage across the capacitor C and hence between the terminals 0,, O rapidly rises to a value well above the peak value of the alternating current supply, the maximum voltage across capacitor C being limited and controlled by the zener effect of the semiconductor switching device TR,. Thus, a series of high voltage pulses of square waveform is produced across the output terminals 0,, during each half cycle of the alternating current supply, of the same polarity as that half cycle.

When the discharge lamp D is connected across the terminals 0,, 0 the voltage across the capacitor C is limited to the ignition voltage of the lamp'D, the lamp igniting very early in the supply voltage half cycle due to the rapid rise in the voltage across capacitor C The capacitor C then discharges into the lamp D, delivering appreciable power. Both the choke L, and the capacitor C therefore act as storage elements, effectively ballasting the lamp D.

Once the lamp D has struck the high frequency oscillations in the network L,, C, are damped by the loading effect of the lamp D and the peaks in the output voltage across the terminals 0,, 0 fall. The voltage across the capacitor C, in the running condition of the lamp D is, however, still sufficiently high to maintain an are discharge voltage across the lamp of the same order as the mains voltage supply. The damping of the network L,, C by the lamp D also ensures that the lamp D is not extinguished after ignition by the reverse voltage swing across capacitor C It will be apparent that waveform of the voltage applied to the lamp D is of reasonably square form since the lamp strikes early in each half cycle of the alternating supply and appreciable power is immediately delivered to the lamp D. Consequently the arrangement operates at a good power factor and efficiency.

To limit the peak currents, which may in certain cases result in shortened lamp life, the circuit of FIG. 1 is modified pursuant to the present invention, as illustrated in FIG. 2, by including a small auxiliary choke L connected between the ballast choke L, and the output terminal 0, so as to be in series with capacitor C between the output terminals 0,, 0

By means of a high frequency choke L inductively coupled to choke L and a capacitor C connected in series between the common point of the ballast choke L, and the switching device TR, and the output terminal 0 a high voltage spike may be produced at the instant of triggering the switching device TR,, for starting the lamp D. particularly where the latter requires a high starting voltage. The high frequency choke L is in practice constituted by a few turns of wire wound over the winding on the auxiliary choke L By suitable adjustment of the turns ratio of the windings of the two chokes L L and/or of the other parameters of the auxiliary circuit, the circuit can be made to produce starting voltage pulses of from I to l0 Kv, or even higher, when operating on a 240 volts main supply.

FIG. 3 illustrates an alternative mode of connection of the circuit components of FIG. 2. The circuit of FIG. 3 has similar operating characteristics to that of FIG. 2. Referring now to FIG. 4, in an alternative preferred embodiment to that illustrated in FIG. 2, the switching device is connected in series with the capacitor C instead of between the choke L, and input terminal I,. In addition the choke L and capacitor C, are connected between common point of capacitor C, and switching device TR, and the output terminal 0 It will be appreciated that this arrangement operates in substantially the same manner as the arrangement of FIG. 2, al-

though the voltage waveforms appearing between output terminals 0, and O in the arrangements of FIGS. 2 and 4 are somewhat different.

The circuit according to the present invention provides a simple semi-choke type ballast which, by virtue of the fact that a substantially square output voltage waveform is produced from an alternating current voltage supply, has increased efficiency compared with conventional discharge lamp operating circuits. The circuit permits lamps with arc voltages approaching that of the mains supply voltage to be operated without a transformer, since the lamp is operated by a substantially square voltage waveform.

The difference between the lamp running voltage and the mains alternating supply voltage is smaller than, in a conventional lamp ballast circuit, so the circuit combination permits a substantial reduction in the magnitude of the series impedance element, that isgthe ballast choke, this being generally the most expensive component in a discharge lamp ballast circuit. The use of a smaller ballast choke and the presence of the capacitor C in the circuit according to the invention also reduces the size of the power factor correction capacitor C, required for the circuit of the present invention.

The circuit of the present invention has particular ap plication to the running of the large range of conventional discharge lamps which have higher are and running voltages from ordinary single phase (i.e. domestic) mains supplies. Hitherto the running of such lamps has necessitated ballast circuits which included bulky high reactance transformers and correspondingly large power factor correction capacitors, or which for economic reasons could be run only from high voltage mains supplies.

Other advantages of the invention are that it will enable discharge lamps of higher wattage rating to be run at the same current rating as existing smaller wattage rating lamps. Also, the lamp operating circuit according to the invention in its practical forms is substantially smaller in size than conventional discharge lamp operating circuits.

Typical practical values for some of the components of the circuits shown in FIGS. 2 and 4 for the operation of a 600 watt high pressure sodium vapour lamp from a 240 volt 50 Hz alternating current mains supply are as follows:

L, mH C, 7 mF C 1.1 mF CQ= OIOI iiiF 7 L turns) wound on a by /2 ferrite core.

lows:

L 400 ml-l C, 4 mF C 0.6 mF

the other components C T,, L and L being as above.

In other arrangements in accordance with the invention, it may be desirable to include a capacitor. C across the switching device T, to limit the rate of rise of voltage across the device T,, a resistor R being connected in series with the capacitor C to limit surge current from the capacitor C back into the switching device T,, when it is triggered into conduction, as shown in FIG. 2. With the particular component values given above, by way of example, however, the presence of the capacitor C and resistor R is found unnecessary.

In a further modification of the arrangements shown in FIGS. 1 to 3, one or more negative temperature coefficient resistors are included in the resistor chain R R R to provide two different operating conditions, one when the lamp D is starting and the other after a predetermined time when the lamp is near, or in, its normal running condition.

It will be understood that the control network for triggering the switching device TR need not necessarily consist of resistors. Thus this network may alternatively comprise, for example, back-to-back zener diodes, trigger diodes, thermistors or a combination of such devices.

I claim:

1. A circuit for operating an electrical discharge lamp from an alternating current supply comprising: a pair of input terminals for connection to an alternating current supply; a pair of output terminals for connection to a discharge lamp; a ballast inductance connected between one of the input terminals and one of the output terminals; a connection between the other input terminal and the other output terminal; a capacitor connected between said other input terminal and the side of said ballast inductance nearer said one output terminal; a semi-conductor switching device connected in series with the ballast inductance and the capacitor between said input terminals; a second-inductance connected between said one output terminal and the ballast inductance; and a control network for rendering the switching device conducting for a predetermined part of each of at least one set of alternate half cycles of an alternating current supply connected across the input terminals, to provide a .voltage across the output terminals for both starting and running a discharge lamp.

, 2. A circuit according to claim 1 wherein the switching device and said second inductance are connected in series with the ballast inductance between said one of the input terminals and said one of the output terminals.

3. A circuit according to claim 1 wherein the switching device and said capacitance are connected in series between said other input terminal and the side of said ballast inductance nearer said one output terminal.

4. A circuit according to claim 1 wherein said switching device is a bilateral device and is rendered conducting by the control network for a predetermined part of each half cycle of the alternating current supply.

5. A circuit according to claim 4 wherein the switching device is a bilateral semiconductor controlled rectitier.

6. A circuit according to claim 1 wherein the ballast inductance and the capacitance form a resonant network having a resonant frequency higher than the frequency of alternating current supply.

7. A circuit according to claim l-wherein a third inductance inductively coupled with said second inductance is connected between an intermediate point of said ballast inductance/switching device/capacitance seriesv combination and said other input terminal,

thereby to produce a high voltage strike for starting a lamp connected between the output terminals.

8. A circuit according to claim 7 wherein said third inductance is connected in series with a second capacitance to form a resonant network having a resonant frequency higher than the frequency of the alternating current supply.

9. A circuit according to claim 1 wherein said control network comprises a resistive voltage divider connected across the input terminals. 

1. A circuit for operating an electrical discharge lamp from an alternating current supply comprising: a pair of input terminals for connection to an alternating current supply; a pair of output terminals for connection to a discharge lamp; a ballast inductance connected between one of the input terminals and one of the output terminals; a connection between the other input terminal and the other output terminal; a capacitor connected between said other input terminal and the side of said ballast inductance nearer said one output terminal; a semi-conductor switching device connected in series with the ballast inductance and the capacitor between said input terminals; a second inductance connected between said one output terminal and the ballast inductance; and a control network for rendering the switching device conducting for a predetermined part of each of at least one set of alternate half cycles of an alternating current supply connected across the input terminals, to provide a voltage across the output terminals for both starting and running a discharge lamp.
 2. A circuit according to claim 1 wherein the switching device and said second inductance are connected in series with the ballast inductance between said one of the input terminals and said one of the output terminals.
 3. A circuit according to claim 1 wherein the switching device and said capacitance are connected in series between said other input terminal and the side of said ballast inductance nearer said one output terminal.
 4. A circuit according to claim 1 wherein said switching device is a bilateral device and is rendered conducting by the control network for a predetermined part of each half cycle of the alternating current supply.
 5. A circuit according to claim 4 wherein the switching device is a bilateral semiconductor controlled rectifier.
 6. A circuit according to claim 1 wherein the ballast inductance and the capacitance form a resonant network having a resonant frequency higher than the frequency of alternating current supply.
 7. A circuit according to claim 1 wherein a third inductance inductively coupled with said second inductance is connected between an intermediate point of said ballast inductance/switching device/capacitance series combination and said other input terminal, thereby to produce a high voltage strike for starting a lamp connected between the output terminals.
 8. A circuit according to claim 7 wherein said third inductance is connected in series with a second capacitance to form a resonant network having a resonant frequency higher than the frequency of the alternating current supply.
 9. A circUit according to claim 1 wherein said control network comprises a resistive voltage divider connected across the input terminals. 